For hard or physical copies of a document, a hand-written signature may be used to demonstrate that an author of the signature has approved or agreed to the content of the document. Under some circumstances, the signature may be used to legally bind the author to act (or not act) in accordance with the contents of a document. For this reason, at least some documents require that the signature be witnessed by a person who can confirm that the author signed the document.
The concept of signing a physical copy of a document has been applied to at least some electronic documents. For example, at least some known mechanisms for signing an electronic signature include inputting a particular sequence of keystrokes associated with a user (e.g., a password, a personal identification number (PIN)). However, an integrity of a password or PIN may be compromised if a person other than the user acquires possession of the password or PIN.